Dangerous levels of radiation have leaked into the atmosphere following a fire and explosions at a nuclear plant in northeastern Japan, officials warned on Tuesday as the country reels in the wake of last week's devastating earthquake and tsunami.

Radiation levels around the Fukushima No.1 plant on the eastern coast had "risen considerably", Naoto Kan, the prime minister said, and his chief spokesman announced the level was now high enough to endanger human health.

The UN's nuclear watchdog, the International Atomic Energy Agency on Tuesday upgraded the crisis to a level-6 "serious accident" on a 1-7 scale. The Chernobyl meltdown in 1986, considered the world's worst nuclear disaster, was a grade-7 incident.

The IAEA's Japanese chief, Yukiya Amano, also warned of "a possibility of core damage" to the reactor.

"The problem is very complicated, we do not have all the details of the information so what we can do is limited," Amano told a news conference. "I am trying to further improve the communication."

In Tokyo, some 250km to the southwest, authorities said radiation levels far higher than normal had been detected in the capital, the world's biggest urban area, but said the increase didn't pose a threat to health.

Kan warned people living up to 10km beyond a 20km exclusion zone around the nuclear plant to stay indoors. Hundreds of thousands of people have already been evacuated from the affected region.

"I would like to ask the nation, although this incident is of great concern, I ask you to react very calmly," he said.

China became the first government to organize a mass evacuation of its citizens from Japan's northeast on Tuesday, while other foreigners joined those moving away from areas threatened by radiation.

Austria said it was moving its embassy from Tokyo to Osaka due to radiation concerns. France recommended that its citizens leave the Japanese capital, while the U.S. government advised Americans to avoid travel to Japan.

The official death toll from last week's twin disasters rose to 3,373, police said on Tuesday, while officials said at least 10,000 were likely to have perished.

Aid workers and search teams from across the world have joined 100,000 Japanese soldiers in a massive relief push in the shattered areas.

Millions have been left without water, electricity, fuel or enough food and hundreds of thousands more are homeless and facing harsh conditions with snow, rain and sub-zero temperatures overnight.

Strong aftershocks

Japan was shaken by more strong aftershocks on Tuesday, prompting buildings to sway in Tokyo. The first, measuring 6.2 in magnitude, struck on Tuesday night off the coast of Fukushima prefecture, 325 kilometers northeast of Tokyo.

Three minutes later, a 6.0-magnitude quake rumbled under Shizuoka prefecture, 90 kilometers southwest of the capital.

"We had an aftershock of about 6-magnitude," said Wayne Hay, Al Jazeera's correspondent in Akita. "I was on the 11th story and certainly the building did sway for about 30 seconds."

Fearful citizens stripped supermarket shelves on Tuesday, prompting the government to warn against panic-buying, saying this could hurt the provision of relief supplies to quake-hit areas.

Scared Tokyo residents filled outbound trains and rushed to shops to stock up on food, water, face masks and emergency supplies amid heightening fears of radiation. At the city's main airports, hundreds of people lined up, many with children, boarding flights out.

Al Jazeera’s Steve Chao, reporting from Yamagata, said a no-fly zone has been established in a 30 km radius over the Fukushima plant.

The fire, which was later extinguished, broke out in the plant's number-four reactor, he said, meaning that four out of six reactors at the facility were in trouble - and temperatures were reportedly rising in the last two.

Damage is 'massive'

Radiation levels later dropped at both the plant and in Tokyo, Yukio Edano, the chief government spokesman, said.

As well as the atomic emergency, Japan is struggling to cope with the enormity of the damage from Friday's record quake and the tsunami that raced across vast tracts of its northeast, destroying all before it.

Japan is the only country in the world to have experienced a nuclear attack - two bombs dropped by the US during World War II killed some 200,000 people - and citizens are gripped by fear of nuclear fallout.

"What we most fear is a radiation leak from the nuclear plant," Kaoru Hashimoto, 36, a housewife living in Fukushima city 80km northwest of the stricken plant, said.

"Many children are sick in this cold weather but pharmacies are closed. Emergency relief goods have not reached evacuation centres in the city. Everyone is anxious and wants to get out of town. But there is no more petrol."

More than 200,000 people have already been evacuated from the exclusion zone around the crippled plant.
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At one shelter, a young woman holding her baby told public broadcaster NHK: "I didn't want this baby to be exposed to radiation. I wanted to avoid that, no matter what."

Social harmony

However, even in evacuation centres filled with quake-shocked and tsunami survivors, Japan's famed emphasis on social harmony is in evidence.

From the sharing of tasks among volunteers to the neat arrangement of shoes outside the living areas, life in the shelters is orderly and peaceful.

The crisis at the ageing Fukushima plant has worsened daily since Friday's quake and tsunami knocked out cooling systems.

On Saturday an explosion blew apart the building surrounding the plant's NO.1 reactor. On Monday, a blast hit the No.3 reactor, injuring 11 people and sending plumes of smoke billowing into the sky.

Early on Tuesday a third blast rocked the No.2 reactor. That was followed by a hydrogen explosion that started a fire at the No.4 reactor.

Edano, the chief government spokesman, said radioactive particulates leaked along with the hydrogen.

The IAEA said Tokyo had asked for expert assistance in the aftermath of the quake, which US seismologists are now measuring at 9.0-magnitude, revised up from 8.9.